Special signal processing can be performed in motor control centers to determine fault conditions of rotating machinery such as a broken or damaged bar in an induction motor, for example. One technique described in G. B. Kliman and J. Stein, "Methods of Motor Current Signature Analysis," Electric Machines and Power Systems, vol. 20, pp. 463-474, 1992, ascertains a broken bar condition by interpreting the power line spectra. The signals that are important for diagnosis are of very small magnitude as compared to the largest sine wave component, which is typically the fundamental (first harmonic) component of the line current occurring at a frequency of about 60 hertz. The frequency location of signals that are used to diagnose a broken bar are dependent on the slip and the synchronous speed (which is a function of the number of poles and the frequency of the largest sine wave component).
A long term fourier transform can be used both to reduce the windowing leakage that results from sampling a time limited waveform and to determine the presence of a frequency component produced by a broken bar. A long term fourier transform, however, has several limitations. For example, the largest sine wave component frequency can vary during a long term average. Furthermore, the motor's load can vary with time, thus altering the slip and spreading the signal of interest over a wider frequency range to reduce its peak power per hertz. Additionally, a large magnitude component requires that a digital signal analysis system quantize data in such a way that the necessary dynamic range is provided. Proposed extensions of these techniques to motor faults other than broken bars requires increased sensitivity and dynamic range on the order of 90-110 decibels.
Demodulation on the 60 Hz carrier (largest component) and extremely narrow band (notch) filtering have been used in attempts to remove the largest sine wave component before signal processing. A limitation of simple demodulation is the potential aliasing of the frequencies below the largest sine wave frequency, and a limitation of notch filtering is the small modulation of the largest sine wave frequency and therefore the differing degrees of attenuation due to the filter's "skirts" resulting in artifact modulations.